German patent application 1 159 180 discloses a polarimeter with a polarizer, an analyzer, and a radiation receiver arranged behind the analyzer in which an optically active sample is inserted between the polarizer and the analyzer, and the polarizer is caused to make rotary oscillations about the beam axis. The output signal of the radiation receiver is supplied to phase-sensitive balancing means which, when the polarization plane rotates, effect a follow-up rotation of the analyzer. In the prior art polarimeter the polarizer is connected to a cylindrical, diametrically magnetized permanent magnet. The permanent magnet is surrounded by concave-cylindrical pole surfaces of an electro-magnet excited by an A.C. voltage. A radial leaf spring fetters the permanent magnet and the polarizer to its position of rest.
Thus, in the prior art driving device, the polarizer oscillates sinusoidally with the main frequency near the resonance of the oscillatory system formed by the polarizer, permanent magnet, and leaf spring. To this end the leaf spring has to be adjusted in its length such that the resonant frequency of the system coincides substantially with the main frequency. This requires an adjustment. In addition, the leaf spring wears out at the clamping location.
In order to keep the oscillation angle constant the driving mechanism has to be stabilized exactly.
The measuring signals have the frequency of the main voltage and the second harmonic of the main frequency. Such signals are easily disturbed by the main voltage. A "boozing noise" occurs. Thereby, the signal processing is complicated.
When the apparatus is switched on, a longer running in time is required because of stability reasons. Therefore, in practical work, the swinging polarizer is also kept switched on when there is no measurement.
Finally, in the prior art driving mechanism the amplitude of the oscillation remains constant. It is also not possible to vary the oscillation amplitude in a defined way without difficulty. When the examined sample has a large angle of rotation, that means when the analyzer transmits much light, the alternating signal obtained with twice the main frequency is small. The sensitivity is low.